1. Field of the Invention
This invention relates to a pipetting device comprising a retaining cone for holding a slip-on pipette tip and having a piston chamber, which communicates with a passage of the retaining cone and permits a piston to reciprocate in said chamber, also comprising a pipette tip and a filter.
2. Description of the Prior Art
In chemistry and bacteriology it is known to use pipetting means in order to prevent a contact of the mouth of the operator with a liquid which is sucked by the mouth. It is known to provide a separate plug, which is fitted and has capillary through passages for separating cells in liquids
It is also known to provide particularly for use with sedimentation measuring tubes a pipetting device which comprises an insert, which is slidably inserted into the top end of a capillary formed in the pipette. That insert opposes the sucking of liquid as far as to a predetermined height and subsequently opposes the further sucking and the return flow of the column of liquid. Plugs made of cotton wool have been used for that purpose and have been inserted into the top end of a pipette in a complicated manner. Because the dimensional stability is low, the length and height of the liquid column cannot be exactly predetermined in that way and it is difficult to remove the plug from the pipette if the latter consists of glass.
German Patent Specification No. 21 55 566 proposes to avoid said disadvantages in that the insert consists of a capillary tube, which fits into the capillary passage of the pipette. The capillary tube has an enlarged head portion, which at its top end contains a filter plug, which presents a high resistance to the flow of liquid but it highly permeable to air. That plug consists of felt, filter material, woven fabric or foam so that it can easily be handled and can be inserted with high accuracy.
German Utility Model No. 1,978,016 proposes that pre-formed filter plugs contained in a magazine can be inserted by means of a push rod into a mouthpiece that is integrally formed on the pipette.
U.S. Pat. No. 4,059,020 discloses filters for use with plastic tips of micropipettes. The filters are made of porous plastic or metal and may be engaged with the pipette tip as the liquid sample is taken so that the latter is filtered at the same time. That filter is only for liquid and is removed when the sample is taken and the filtered liquid is to be dispensed. It has been stated that tips which differ in size and shape may be used. The filter device consists of a holder, which is adapted to be slipped on the pipette tip and contains a filter and has a passage which is cylindrical or conical at the slip-on end so that an adaptation to pipette tips differing in shape is permitted. The porous filter for liquid is aligned with the flow passage and is contained in a filter chamber that is enlarged relative to the flow passage.
U.S. Pat. No. 4,461,328 discloses a pipetting device that comprises one or more pipette tubes, to which a hydrophobic filter paper is secured. That paper restricts the rise of an aqueous liquid in each tube so that a predetermined quantity of liquid may be held in each tube. In a multiple pipette, said filter means permits an exactly predetermined quantity of liquid to be sucked into each pipette tube by the operation of a simple piston means.
Hydrophobic filters used for that purpose are freely permeable to air or other gases but require a relatively high pressure to be applied for a flow of an aqueous liquid into and through the filter. It is also known to use such filters for a filtration of air or for a passage of air from a liquid mass.
Hydrophobic filters may consist of paper or plastic and have porelike passages, which act as capillaries. Typical plastics consist of polytetrafluoroethylene, polyvinylchloride and halogenated fluoroalkali. In the known design the filter is mainly used to restrict the quantity of liquid which can be sucked into pipette tubes whereas a flow of air or other gases for permitting a taking or dispensing of liquid is permitted. The pipetting device specifically consists of a trough, which is provided with a plurality of pipette tubes, on which a common filter sheet is disposed.
In connection with the use with pipettes which are operable by suction, it is also known to protect the operator of the pipette by the provision of an adapter, which is interposed between the pipette and the mouthpiece and has an interior space, which contains a filter membrane having pores which are so small that a passage of bacteria will be prevented.
U.S. Pat. No. 3,995,496 discloses for use with pipettes a mouth-protecting device consisting of a plastic tube, which contains compressed fibers, which constitute a liquid barrier. In contact with a liquid said fibers will expand so that the flow of liquid through the device will be prevented when said liquid is incidentally sucked beyond the end of the pipette. The fibers consist preferably of cotton.
U.S. Pat. No. 4,483,825 discloses a pipette which serves to handle a plasma that has been formed from centrifuged blood. The pipette contains a filter for liquid, which is permeable only to plasma and is impermeable to blood cells.
British Patent Specification No. 1,592,855 discloses a different pipette, which is used, e.g., for handling blood and has a parallel bored passage which contains a porous plug that constitutes a liquid barrier for limiting the sample volume and for moistening the sample in the passage with liquid so that the sample will be maintained in a liquid state. The plug is a dimensionally stable body, which has open pores and which is conical at least before it is inserted into the passage so that its insertion will be facilitated. The plug preferably consists of sintered polyethylene and owing to its conicity is mechanically held in the bore. That design is intended to avoid problems which have arisen with cotton plugs.